To understand why some galaxies burst while others do not, Dr David Meier of the National Radio Astronomy Observatory and the New Mexico Institute of Mining and Technology and his colleagues from Canada, the United States, and Germany, used the Atacama Large Millimeter/submillimeter Array to dissect a cluster of stellar nurseries in the center of NGC 253.
New ALMA data reveal a diffuse envelope of carbon monoxide gas (shown in red), which surrounds regions of active star formation (yellow); by dissecting these regions with this powerful radio telescope, astronomers are uncovering clues to the processes and conditions that drive furious star formation; the ALMA data are superimposed on a Hubble image. Image credit: B. Saxton / NRAO / AUI / NSF / ALMA / ESO / NAOJ / A. Leroy / STScI / NASA / ST-ECF / ESA / CADC / NRC / CSA.
NGC 253, also known as the Silver Coin, Silver Dollar Galaxy or Sculptor Galaxy, was discovered on September 23, 1783 by Caroline Herschel (sister of William Herschel).
The galaxy is located approximately 11.5 million light-years away and is the brightest member of the Sculptor Group of galaxies.
It is considered a starburst galaxy, where stars form and explode at an unusually high rate.
“All stars form in dense clouds of dust and gas. Until now, however, astronomers struggled to see exactly what was going on inside starburst galaxies that distinguished them from other star-forming regions,” said Dr Adam Leroy of the Ohio State University in Columbus, who is a co-author of the paper accepted for publication in the Astrophysical Journal (arXiv.org preprint).
“The Atacama Large Millimeter/submillimeter Array (ALMA) changes that by offering the power to resolve individual star-forming structures, even in distant systems.”
ALMA’s exceptional resolution and sensitivity allowed Dr Meier, Dr Leroy and their colleagues to identify 10 distinct star-forming regions inside the heart of NGC 253.
The astronomers then mapped the distribution of about 40 millimeter-wavelength ‘signatures’ from different molecules at the galaxy’s core.
This was critically important since different molecules correspond to different conditions in and around star-forming clouds.
For example, carbon monoxide corresponds to massive envelopes of less dense gas that surround stellar nurseries.
Other molecules, like hydrogen cyanide, reveal dense areas of active star formation. Still rarer molecules, like H13CN and H13CO+, indicate even denser regions.
By comparing the concentration, distribution, and motion of these molecules, the scientists were able to peel apart the star-forming clouds in the galaxy, revealing that they are much more massive, 10 times denser, and far more turbulent than similar clouds in normal spiral galaxies.
These differences suggest that it’s not just the number of stellar nurseries that sets the throttle for a galaxy to create new stars, but also what kind of stellar nurseries are present.
Because the star-forming clouds in NGC 253 pack so much material into such a small space, they are simply better at forming stars than the clouds in a galaxy like the Milky Way.
Starburst galaxies, therefore, show real physical changes in the star-formation process, not just a one-to-one scaling of star formation with the available reservoir of material.
“These differences have wide-ranging implications for how galaxies grow and evolve,” Dr Leroy said.
“What we would ultimately like to know is whether a starburst like NGC 253 produces not just more stars, but different types of stars than a galaxy like the Milky Way.”
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David S. Meier et al. 2015. ALMA Multi-line Imaging of the Nearby Starburst Galaxy NGC 253. ApJ, accepted for publication; arXiv: 1501.05694
